Colonization with potentially pathogenic organisms and infections continue to be seen despite the use of selective decontaminating (anaerobic preserving) oral prophylactic antibiotics during periods of profound, persistent granulocytopenia in patients with acute leukemia undergoing induction chemotherapy. This may be related to the use of systemic antibiotics which suppress alimentary canal anaerobes which are believed to be responsible, in large measure, for decreasing/resisting colonization. The aim of this project is to determine if consistent alimentary canal anaerobic flora preservation with concurrent suppression of aerobic potential pathogens will reduce colonization with new, potentially pathogenic microorganisms in acute leukemia patients undergoing induction chemotherapy. Patients will receive selective decontaminating but anaerobic sparing prophylactic oral antibiotics, being randomly assigned to amphotericin B plus either trimethoprimsulfamethoxazole or norfloxacin. In the setting of fever and granulocytopenia, however, these patients will be randomized to receive therapeutic antibiotic(s) that eithr continue to preserve anaerobes (ceftazidime +/- tobramycin) or have anti-anaerobic activity (ceftazidime + piperacillin). The effect of prohylactic and systemic antibiotic(s) on alimentary canal flora will be confirmed by quantitative stool cultures and determinations of Beta-aspartylglycine in faecal specimens in ten patients in each of the groups receiving either mode of prophylaxis and either form of systemic therapy. Microbiological surveillence with twice weekly semiquantitative gingival and rectal cultures of all patients throughout the study period will permit determinations of colonization. A comparison of new organism acquisition, particularly gram-negative and/or antibiotic resistant potential pathogens and fungi in patient groups in whom alimentary canal anaerobes were or were not consistently preserved throughout the period of granulocytopenia, will test the concept that alimentary canal anaerobic flora preservation reduces colonization, thereby imparting colonization resistance.